There are commercially available today telephone instruments for both business and residential use which enable a user to automatically and repeatedly redial a number when a busy signal has been obtained for that number. The redialing system for such an instrument generally comprises three principal components; a call progress decoder, a redial memory and a microprocessor for interacting between the decoder and the memory to cause the memory to redial the desired number at appropriate intervals.
A call progress decoder monitors the tones transmitted over the telephone line so as to distinguish between and generate unique output signals upon detecting a dial tone signal, a ringing signal, a reorder signal and a busy signal. Certain decoders distinguish the various tones on the basis of tone frequency while others respond to the timing of interruptions of the various tones. In either case, the call progress decoders are usually comprised of packaged CMOS transistor integrated circuits which are both small in size and relatively inexpensive.
The redial memory system also can be obtained inexpensively in package form.
The microprocessor used to interrelate the decoder and the memory often is of considerable complexity and can be costly. Indeed, the cost of the microprocessor places the automatic redialing system out of the reach of many ordinary residential telephone owners.
The present invention eliminates the necessity for using a microprocessor in an automatic redialing system by substituting therefor a simple digital logic circuit.